Thermodynamics of a Ca2+-dependent highly thermostable alkaline protease from a haloalkliphilic actinomycete

An alkaline protease from salt-tolerant alkaliphilic actinomycetes, Nocardiopsis alba OK-5 was purified by a single-step hydrophobic interaction chromatography and characterized. The purified protease with an estimated molecular mass of 20 kDa was optimally active at 70 °C in 0-3 M NaCl and 0-100 mM Ca2+ displaying significant stability at 50-80 °C. The enzyme was stable at 80 °C in 100 mM Ca2+ with Kd of 17 × 10−3 and t1/2 of 32 min. The activation energy (Ea), enthalpy (ΔH*), and entropy (ΔS*) for the protease deactivation calculated in the presence of 200 mM Ca2+ were 38.15 kJ/mol, 35.49 kJ/mol and 183.48 J/mol, respectively. The change in free energy (ΔG*) for protease deactivation at 60 °C in 200 mM Ca2+ was 95.88 kJ/mol. Decrease in ΔH* reflected reduced cooperativity of deactivation and unfolding. The enzyme was intrinsically stable that counteracted heat denaturation by a weak cooperativity during the unfolding. Further, the enzyme was highly stable in the presence of various cations, surfactants, H2O2, β-mercaptoethanol, and commercial detergents. The compatibility of the enzyme with various cations, surfactants, and detergent matrices suggests its suitability as an additive in the detergents and peptide synthesis.